1. Field of the Invention
The present invention relates generally to a control system for an automotive automatic power transmission. More specifically, the invention relates to an automatic power transmission control system applicable for an automatic power transmission having a power transmission system which includes an one-way clutch for reducing shock upon effecting engine braking. Further specifically, the invention relates to an automatic power transmission control system for controlling timing of delivery of negative torque for an automotive engine for deceleration.
2. Description of the Background Art
It is common among modern automatic power transmissions to provide a one-way clutch for permitting engine driving torque to be delivered to the final drive and blocking negative driving torque from the final drive to the engine. Therefore, in order to make the negative torque effective for decelerating the engine, an overrun clutch is provided in a parallel relationship with the one-way clutch so that the negative driving torque is input to the engine therethrough. In order to operate the automatic power transmission, it therefore becomes necessary to appropriately control a timing to make the overrun clutch active for optimization of the engine braking effect without causing substantial shock which otherwise causes substantial degradation of vehicular driving feeling.
One of the conventionally known engine brake control systems for automatic power transmissions has been illustrated in "NISSAN SERVICE MANUAL" Vol. No. 578, issued by Nissan Motor Co., Ltd. on June, 1987, pages C-36 to C-38. In this prior system, an active range of an overrun clutch, in which the overrun clutch is engaged, is set at a throttle valve open degree of 1/16 or smaller at a transmission operational range of D.sub.1 (first speed at D-range), D.sub.2 (second speed at D-range) and D.sub.3 (third speed at D-range) ranges and while an over-drive (OD) switch is held OFF. Also, the overrun clutch active range is set at the throttle valve open degree of 1/16 or smaller at 2.sub.1 (first speed at 2-range) and 2.sub.2 (second speed at 2-range) ranges, and at an overall throttle valve open degree at 1.sub.1 (first speed at 1-range) and 1.sub.2 (second speed at 1-range) ranges.
It can be appreciated that, in the prior proposed system, the 1-range and the 2-range are used for effecting engine braking in order to decelerate the vehicular traveling speed. In these engine braking effective ranges, if the 2-range is selected, the overrun clutch is engaged only when the throttle valve open degree is maintained smaller than or equal to 1/16 relative to the throttle valve full open position. Therefore, at a partial throttle position having a throttle open degree greater than 1/16, engine braking may not be active at the 2-range to place the vehicle in a coasting condition irrespective of the driver's demand for deceleration. On the other hand, at a part throttle condition in the vicinity of 1/16 of the throttle valve open degree, the overrun clutch is engaged when the throttle valve open angle decreases across 1/16, and engine braking becomes effective at an unexpected timing to surprise the driver and thus cause degradation of the vehicular driving feeling.
It may be possible to establish an engagement of the overrun clutch in an overall throttle valve angular range in order to avoid coasting. However, in such case, the overrun clutch is engaged instantly in response to selection of 2-range down from the D-range, and the overrun clutch as engaged may cause interlocking with a speed ratio shifting clutch (i.e., a high clutch) provided in tandem with the overrun clutch, to cause substantial shock. On the other hand, it may also be possible to set a predetermined delay time between selection of the engine braking range and engagement of the overrun clutch. In such case, the lag time is irrespective of the vehicle driving condition, such as vehicle speed, throttle valve open degree and so forth, so engagement timing cannot be optimal. Namely, in a certain vehicle driving condition, such lag time may be too short to cause premature engagement at an excess speed of the vehicle to cause a substantial shift shock. Also, in certain a case, the overrun clutch is engaged after the power transmission system is placed at a neutral condition to cause an unacceptable shock.